1. Field of the Invention
The present invention relates to a bicycle rear derailleur for shifting a chain from one sprocket to another of a multiple freewheel.
2. Description of the Background Art
As is well known, a bicycle rear derailleur is provided adjacent to a freewheel for shifting a chain from one sprocket to another sprocket of the freewheel. A typical prior art rear derailleur is disclosed in U.S. Pat. No. 4,610,644 for example.
Specifically, the prior art derailleur of the above U.S. patent comprises a chain guide and a pantograph linkage assembly for moving the chain guide axially of the rear hub shaft on which the multiple freewheel is rotatably mounted. The chain guide rotatably supports an upper guide wheel and a lower tension wheel. The pantograph linkage assembly comprises a base member pivotally mounted to a rear fork end of the bicycle frame by a first pivot shaft, inner and outer links having their respective base ends pivotally connected to the base member by first and second pins, and a movable member pivotally connected to the respective free ends of the inner and outer links by third and fourth pins. The movable member pivotally supports the chain guide by a second pivot shaft. The first to fourth pins are located at the four corners of a parallelogram.
The base member of the pantograph linkage assembly is pivotally biased rearward by a first coil spring mounted on the first pivot shaft. Similarly, the chain guide is pivotally biased rearward by a second coil spring mounted on the second pivot shaft. Thus, the chain in engagement with the guide and tension wheels of the chain guide is tensioned by the first and second coil springs. The derailleur incorporating the two chain tensioning springs is said to have a double-tension mechanism.
Further, each of the first to fourth pins of the pantograph linkage assembly is inclined so that its lower end is located laterally outwardly from its inner end. As a result, when the pantograph linkage assembly is deformed to move the chain guide laterally inward, the chain guide also moves downwardly. Due to the inclination or slanting of the pins, the pantograph linkage assembly is referred to as a "slant pantograph linkage assembly".
The prior art derailleur having the slant pantograph linkage assembly and the double tension mechanism has been found to work well with respect to a certain range of freewheels if the first and second coil springs are properly adjusted in their respective spring force. However, the prior art derailleur is still disadvantageous in the following points.
Since the sprockets of the freewheel diametrically increase inward axially of the hub shaft (i.e., the freewheel), the guide wheel of the chain guide need to move downward as it moves inward axially of the hub shaft. On the other hand, the chain tension progressively increases as the chain shifts to a diametrically larger sprocket of the freewheel, so that the chain tension tends to pivotally move the entirety of the derailleur (including the guide wheel) upward about the first pivot shaft against the first coil spring. Thus, due to the increasing chain tension, the guide wheel may actually move upward toward a diametrically larger sprocket despite the fact that the slant pantograph mechanism itself tends to move the guide wheel downward toward a diametrically larger sprocket of the freewheel. As a result, the prior art derailleur may fail to properly shift the chain from a smaller sprocket to a larger sprocket. This disadvantage will be particularly remarkable when a diametrical difference between the largest and smallest sprockets is large.
The above disadvantage itself may be overcome by increasing the elastic force of the first coil spring. However, this solution gives rise to a new disadvantage that the guide wheel becomes excessively far from a diametrically smaller sprocket (due to forcible rearward pivoting of the base member under the strong elastic force of the first coil spring) when the chain shifts thereto from a diametrically larger sprocket. Further, if the first coil spring is made to have an increased elastic force, the chain tension will inevitably increase, consequently hindering smooth shifting of the chain.
Moreover, the double tension mechanism is also disadvantageous in that the elastic forces of the two coil springs must be adjusted in relation to each other for providing a good chain shifting performance with respect to every particular freewheel.